Heterotachy refers to variations in lineage-specific evolutionary rates over time. In the field of
molecular evolution
Molecular evolution is the process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics ...
, the principle of heterotachy states that the substitution rate of sites in a
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
can change through time.
It has been proposed that the positions that show switches in substitution rate over time (that is, heterotachous sites) are good indicators of functional divergence. However, it appears that heterotachy is a much more general process, since most variable sites of homologous proteins with no evidence of functional shift are heterotachous.
The
covarion hypothesis is a specific form of heterotachy. Some studies have proposed functional divergence models that are also heterotachous. Additionally, some
mixture models
In statistics, a mixture model is a probabilistic model for representing the presence of subpopulations within an overall population, without requiring that an observed data set should identify the sub-population to which an individual observation ...
that do not explicitly account for rate-shift, but site-partitions evolving at different relative substitution rates across
lineages are mathematically heterotachous.
Failure to take heterotachy into account in phylogenetic reconstructions may lead to incorrect
phylogenetic trees. Thus Zhong et al. (2011) say that heterotachy is one of the reasons for variability in reconstructions of the origin of
gnetophyte
Gnetophyta () is a division of plants (alternatively considered the subclass Gnetidae or order Gnetales), grouped within the gymnosperms (which also includes conifers, cycads, and ginkgos), that consists of some 70 species across the three re ...
s.
References
{{Reflist
Molecular evolution